Friction stir welding apparatus

ABSTRACT

A body member and flange members are butted together at first and second abutment portions, and are retained by first and second backing jigs. In this arrangement, inner circumferential surfaces of the first and second abutment portions are fitted on and in close contact with outer circumferential surfaces of the first and second backing jigs. Subsequently, the friction stir welding is performed along each of the first abutment portion and the second abutment portion by using a probe which is rotated at a high speed.

RELATED APPLICATIONS

This application is a divisional application Ser. No. 10/758,071, filedJan. 14, 2004, now U.S. Pat. No. 7,137,545 which claims priority toJapanese Patent Application No. 2003-006141 filed Jan. 14, 2003. Thecontents of the aforementioned applications are hereby incorporated byreference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a method of and an apparatus forfriction stir welding for joining an abutment portion by moving arotating probe relatively along the abutment portion while pressing theprobe against one surface of the abutment portion at which an end of afirst cylindrical member and an end of a second cylindrical member arebutted together.

2. Description of the Related Art

The friction stir welding (hereinafter referred to as “FSW” as well) isgenerally known, in which two workpieces are joined in solid phase byutilizing the frictional heat which is generated when a rotating probeis inserted into the workpieces (objects). When the joining is performedas described above, the joined portion of the workpiece can maintain astrength which is about 80% of that of the base material, and it is alsopossible to prevent crystals from becoming coarse.

If the welding is performed by means of the conventional spot welding orthe conventional electron beam welding by using an aluminum material asthe workpiece, for example, some excessive heat is applied to thealuminum material. Therefore, the strength may be decreased due todeterioration of the material and/or change into coarse microstructure.

In contrast, the joining by FSW is performed at about 500° C. even whena metal material such as the aluminum material having a relatively lowmelting point (about 600° to 660° C. in the case of the aluminummaterial) is used. Therefore, it is possible to perform FSW by usingother materials such as magnesium, titanium, and polymer as well as thealuminum material.

As an example of the application of FSW to the aluminum material, analuminum frame of a large member such as an electric train may beexemplified. In the case of such a large member, the thickness of thealuminum material is usually not less than 5 mm considering the strengthof the joining. On the other hand, if light weight is desired as well asthe improvement of strength in the case of gas turbine engines or thelike, it is difficult to increase the thickness of each component. Forthis reason, for example, a thin plate aluminum material of about 1.2 mmthickness is used to prepare an outer frame of the gas turbine engine.

However, when an abutment portion, at which both ends of thinplate-shaped aluminum materials are butted together, is joined by meansof FSW to form a cylindrical member having a relatively large diameter,it is impossible to obtain a satisfactory circularity or roundnessbecause the aluminum material is thin.

Further, when an abutment portion, at which ends of two cylindricalmembers are butted together, is joined by FSW, the circumferentiallengths of the respective ends may not be the same. Therefore, if thecylindrical members are joined in this state by FSW, the difference inphase appears at the final position of the joint, and for example, theshape is deformed in a wavy form, resulting in wrinkles or corrugations.

Additionally, the pressing force of 1 t to 2 t is applied to the thinplate-shaped aluminum material during the joining by FSW. Therefore,some irregularity may appear at the joined portion as a result of FSW,if the aluminum material is not retained reliably.

Japanese Laid-Open Patent Publication No. 11-226759 describes a methodof joining aluminum members as shown in FIG. 9, for example. A backingmember 3 of the same material as that of hollow tubes 1 a, 1 b, isarranged at the inside of an abutment portion 2 of the hollow tubes 1 a,1 b made of aluminum. A predetermined interstice or clearance C isformed between the backing member 3 and the inner surface of theabutment portion 2.

In this arrangement, a probe tip 4 a of a probe 4 is inserted into theabutment portion 2 and the backing member 3 while the probe 4 is rotatedat a high speed, and the probe 4 is moved along the abutment portion 2.Accordingly, the entire circumference of the circumscribing abutmentportion 2 is welded by the friction stir welding.

In the case of Japanese Laid-Open Patent Publication No. 11-226759, thebacking member 3 is provided as a reinforcing member of the hollow tubes1 a, 1 b, and the reinforcing member is joined to the inner surfaces ofthe hollow tubes 1 a, 1 b. However, the hollow tubes 1 a, 1 b, to whichthe backing member 3 is joined as described above, cannot be used as anouter frame of a gas turbine engine, for example. Further, theinterstice C is provided between the backing member 3 and the innersurfaces of the hollow tubes 1 a, 1 b. Therefore, it is impossible tomaintain the circularity of the abutment portion 2 especially when thehollow tubes 1 a, 1 b are thin and each of them has a cylindrical shapewith a large diameter.

SUMMARY OF THE INVENTION

A principal object of the present invention is to provide a method ofand an apparatus for friction stir welding in which first and secondcylindrical members, which are especially thin and large in diameter,can be joined in a well-suited manner while maintaining the circularityof them, and also it is possible to simplify the arrangement and thesteps.

According to the present invention, a backing jig is externally fittedon first and second cylindrical members. Ends of the first and secondcylindrical members are butted together at an abutment portion. Theabutment portion has one surface (outer circumferential surface) andanother surface (inner circumferential surface) which is fitted on andin close contact with an outer circumferential surface of the backingjig.

In this state, a rotating probe is moved relatively along the abutmentportion while pressing the probe against the one surface of the abutmentportion. Accordingly, the entire circumference of the abutment portionis joined. In this situation, the abutment portion is in close contactwith the outer circumferential surface of the backing jig. Therefore,even when the abutment portion is thin and has a large diameter, it ispossible to maintain the abutment portion to have a desired shape.Therefore, the friction stir welding operation is efficiently performedfor the abutment portion in accordance with the simple and economicalarrangement and steps.

Subsequently, the backing jig is separated from the other surface of theabutment portion after performing the friction stir welding for theentire circumference of the abutment portion. Accordingly, it ispossible to obtain the thin abutment portion by the friction stirwelding. Further, it is possible to avoid occurrence of the residualstress in the abutment portion.

The first and second cylindrical members are externally fitted on thebacking jig while the first and second cylindrical members arerelatively expanded as compared with the backing jig. For example, whenthe first and second cylindrical members are heated, the first andsecond cylindrical members are thermally expanded, and the innercircumferential diameters are increased. Therefore, the first and secondcylindrical members are externally fitted on the backing jig with ease.Further, the first and second cylindrical members are shrunk whencooled, and reliably in close contact with the outer circumferentialsurface of the backing jig.

In this configuration, the outer circumferential surface of the backingjig has a completely circular shape. The ends of the first and secondcylindrical members, which are in close contact with the outercircumferential surface, have an identical circumferential length.Therefore, even when the abutment portion is especially thin with arelatively large diameter, no difference in phase arises, which would beotherwise caused, for example, by deformation and wrinkles when theprobe is inserted. Thus, it is possible to maintain the circularity ofthe abutment portion suitably, and the dimensional accuracy is improved.Further, it is possible to effect correct positioning by avoidingdeviation of the abutment portion, and the friction stir welding iscarried out efficiently.

The backing jig can be separated from the first and second cylindricalmembers easily and reliably, because the first and second cylindricalmembers are relatively expanded as compared with the backing jig.

The first and second cylindrical members are welded by the friction stirwelding along the abutment portion while a pressing force is applied ina direction substantially perpendicular to a direction of insertion ofthe probe. Therefore, it is possible to reliably join the ends of thefirst and second cylindrical members under the pressure, and it ispossible to highly accurately join the abutment portion.

A clamp jig is arranged on the one surface of the abutment portion andprevents the abutment portion from being deformed during the frictionstir welding. Accordingly, the position of the abutment portion is notdeviated when the abutment portion is expanded during the friction stirwelding. Thus, the joining operation is performed highly accurately withease.

The above and other objects, features, and advantages of the presentinvention will become more apparent from the following description whentaken in conjunction with the accompanying drawings in which a preferredembodiment of the present invention is shown by way of illustrativeexample.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 schematically shows a gas turbine engine of an airplane intowhich a fan duct is incorporated, wherein a friction stir welding methodaccording to an embodiment of the present invention is carried out;

FIG. 2 shows a duct structure of the fan duct;

FIG. 3 is a partial exploded perspective view illustrating a frictionstir welding apparatus according to an embodiment of the presentinvention for joining the duct structure;

FIG. 4 is a sectional view illustrating the friction stir weldingapparatus;

FIG. 5 is a magnified view illustrating a part of the friction stirwelding apparatus shown in FIG. 4;

FIG. 6 is a flow chart of the friction stir welding method;

FIG. 7 shows the operation to be performed when a first abutment portionis joined;

FIG. 8 shows the operation to be performed when a second abutmentportion is joined; and

FIG. 9 shows a conventional method of joining aluminum members.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 schematically shows a gas turbine engine 12 of an airplane intowhich a fan duct 10 is incorporated, wherein a friction stir weldingmethod according to an embodiment of the present invention is carriedout.

The gas turbine engine 12 is provided with a fan 14. The fan 14 isrotated at a high speed to suck the air from the outside. The air iscompressed, and it is fed to the backward. A fan bypass passage 18 isformed by a core duct 16 and the fan duct 10 in the vicinity of the fan14. A thrust force is generated for an unillustrated airframe by the airwhich is jetted backwardly through the fan bypass passage 18.

The fan 14 constitutes a low pressure compressor 20. The air compressedby the low pressure compressor 20 is fed to a high pressure compressor22 disposed backward. The air compressed by the high pressure compressor22 is further fed to a combustion chamber 24 disposed backward. Thecombustion chamber 24 is provided with a fuel nozzle 26. Fuel is fedfrom the fuel nozzle 26 to the combustion chamber 24. An air-fuelmixture obtained by mixing the compressed air fed from the high pressurecompressor 22 and the fuel injected from the fuel nozzle 26 is ignitedand burned in the combustion chamber 24 upon the start-up of the engine.

The combustion of the air-fuel mixture produces high temperature andhigh pressure gas. The high temperature and high pressure gas is fed toa high pressure turbine 28 to rotate the high pressure turbine 28 at ahigh speed. The high pressure turbine 28 rotates a rotor 14 a of the fan14, while the high temperature and high pressure gas is fed to a lowpressure turbine 30 after the gas drives and rotates the high pressureturbine 28. The low pressure turbine 30 rotates the fan 14 and the rotor14 a of the low pressure compressor 20.

A starter generator 32, into which a starter and a generator areincorporated, is attached to an outer lower surface of the gas turbineengine 12 by an accessory gear box 34.

FIG. 2 shows a duct structure 40 of the fan duct 10. The duct structure40 comprises a body member (first cylindrical member) 42 which isobtained by forming a thin aluminum plate into a substantiallycylindrical shape, and flange members (second cylindrical members) 44,46 each of which is obtained by forming a thin aluminum plate into asubstantially cylindrical shape. An end 42 a of the body member 42 abutsan end 44 a of the flange member 44 at a first abutment portion 48 whilean end 42 b of the body member 42 abuts an end 46 a of the flange member46 at a second abutment portion 50. The first and second abutmentportions 48, 50 are subjected to the friction stir welding on theirouter circumferential surfaces (first surfaces) 48 a, 50 a to join thebody member 42 and the flange members 44, 46.

FIG. 3 shows a partial exploded perspective view illustrating a frictionstir welding apparatus 60 according to an embodiment of the presentinvention for performing the friction stir welding for the ductstructure 40. FIG. 4 shows a sectional view illustrating the frictionstir welding apparatus 60, and FIG. 5 shows a magnified viewillustrating a part of the friction stir welding apparatus 60.

The friction stir welding apparatus 60 is provided with a pedestalmember 64 which is rotatable while being fixed to a rotary table 62 andwhich fixes the body member 42 and the flange members 44, 46 previouslyjoined to one another temporarily. A substantially disk-shaped supportbase 66 is secured over the pedestal member 64. A support pillar 68 isprovided at a central portion of the support base 66 and extends in thevertical direction (directions of the arrow A). A screw section 72 isformed at the end of the support pillar 68.

A pressing mechanism 70 includes the screw section 72 and a pressingplate 74. The pressing plate 74 is substantially disk-shaped, and has ahole 76 formed at a central portion for inserting the support pillar 68thereinto. The pressing plate 74 applies pressing force to the bodymember 42 and the flange members 44, 46 arranged on the support base 66in directions of the arrow A substantially perpendicular to a directionof insertion of a probe (direction of the arrow B) as described lateron.

A pressing block 77 is engaged with the central portion of the pressingplate 74. A nut member 78 screwed with the screw section 72 presses thepressing plate 74 in the directions of the arrow A by the pressing block77. A hanging bolt 79 is screwed to the end of the screw section 72. Thepressing plate 74 is formed with a plurality of openings 80 which areseparated from each other by equal angles, a plurality of openings 82which have diameters smaller than those of the openings 80, and aplurality of bolt-inserting holes 84. Hanging bolts 85 are screwed tothe pressing plate 74.

A ring section 86 is formed for the support base 66 so that the ringsection 86 is expanded in the directions of the arrow A around thesupport pillar 68. A first backing jig 88 is detachably provided on theouter circumference of the ring section 86 with a predeterminedinterstice C1. A second backing jig 92 is attached by rods 90 which arefixed to the ring section 86.

The first backing jig 88 is formed of, for example, an iron-basedmaterial, and it is substantially ring-shaped. An outer circumferentialsurface 88 a of the first backing jig 88 is completely circular, whichis arranged to be capable of supporting the inner circumferentialsurface (second surface) 48 b of the first abutment portion 48 betweenthe body member 42 and the flange member 44 (see FIG. 5). Thepredetermined interstice C1 is formed between an inner circumferentialsurface 88 b of the first backing jig 88 and the outer circumferentialsurface of the ring section 86.

Screw holes 94 separated from each other by predetermined angularintervals are formed in the back surface (surface opposed to the supportbase 66) of the first backing jig 88. Bolts 96 are screwed into therespective screw holes 94 from the back surface of the support base 66,and thus the first backing jig 88 is fixed onto the support base 66. Aplurality of hanging bolts 98 are installed to the upper surface of thefirst backing jig 88.

Screw holes 100 separated from each other by predetermined angularintervals are formed on the upper surface of the ring section 86. Screwsections 102 have first ends provided with the respective rods 90 arescrewed with the screw holes 100. Screw holes 104 are formed at secondends of the rods 90. Bolts 108 are inserted into holes 106 which areformed through the second backing jig 92. When the bolts 108 are screwedinto the screw holes 104, the second backing jig 92 is fixed to the rods90.

The second backing jig 92 is formed of, for example, an iron-basedmaterial, and is substantially disk-shaped. An outer circumferentialsurface 92 a of the second backing jig 92 is completely circular, whichis in close contact with an inner circumferential surface (secondsurface) 50 b of the second abutment portion 50 between the body member42 and the flange member 44 (see FIG. 5). Taking the spring back of theduct structure 40 into consideration, it is necessary that the ductstructure 40 is maintained to a designed size with the outercircumferential surfaces 88 a, 92 a of the first and second backing jigs88, 92. The outer diameters of the outer circumferential surfaces 88 a,92 a are larger than the inner diameters of the first and secondabutment portions 48, 50 before being heated.

Screw holes 110, which correspond to the holes 84 of the pressing plate74, are formed for the second backing jig 92. Bolts 112 to be insertedinto the holes 84 are screwed into the screw holes 110, and thus thepressing plate 74 is fixed to the second backing jig 92. Hanging bolts113 are screwed to the second backing jig 92.

A first clamp jig 114, which fixes the first abutment portion 48, isarranged on the outer circumferential surface 48 a of the first abutmentportion 48. A second clamp jig 116, which fixes the second abutmentportion 50, is arranged on the outer circumferential surface 50 a of thesecond abutment portion 50.

As shown in FIG. 3, the first clamp jig 114 is a belt obtained byforming a square member to have a ring-shaped configuration. A screwhole 118 is formed at one end surface of the first clamp jig 114, and ahole 120 is formed at the other end surface. When a bolt 122 is screwedinto the screw hole 118 through the hole 120, the size of the firstclamp jig 114 in the radial direction is reduced to tighten the outercircumferential surface 48 a of the first abutment portion 48.

As shown in FIGS. 3 to 5, a plurality of rods 126 are fixed to outercircumferential edge portions of the support base 66 by bolts 124. Therespective rods 126 extend in the directions of the arrow A. Anattachment plate 130 is provided by bolts 128 which are screwed intoends of the respective rods 126. The attachment plate 130 issubstantially ring-shaped. The second clamp jig 116 is fixed to theattachment plate 130 by bolts 132. The second clamp jig 116 issubstantially ring-shaped, which tightens the outer circumferentialsurface 50 a of the second abutment portion 50.

As shown in FIG. 5, a welding machine 140 for joining the first abutmentportion 48 is provided with a rotary tool 142. A probe 144, whichprotrudes by a predetermined length from the end surface, is provided atthe tip of the rotary tool 142. The probe 144 is inserted into the firstabutment portion 48 by a depth which is smaller than the thickness ofthe first abutment portion 48. The second abutment portion 50 may bejoined by using the welding machine 140 described above, oralternatively, by using another welding machine.

An explanation will be made below with reference to a flow chart shownin FIG. 6 about the operation of the friction stir welding apparatus 60and the friction stir welding method according to the embodiment of thepresent invention.

First, the body member 42 and the flange members 44, 46, each of whichhas the cylindrical shape, are manufactured (Step S1). Specifically, asshown in FIG. 2, a thin aluminum plate is formed to have thesubstantially cylindrical shape. The friction stir welding (FSW) isperformed along an abutment portion 42 c at which both ends of the thinaluminum plate are butted together, and thus the abutment portion 42 cis joined. As a result, the body member 42 is obtained.

Similarly, thin aluminum plates are formed to have the substantiallycylindrical shapes, and then respective abutment portions 44 c, 46 c isjoined by the friction stir welding. Thus, the flange members 44, 46 areobtained.

During the friction stir welding described above, an unillustrated probeis arranged while being offset by a predetermined distance from thecenter of the cylindrical shape so that the joined surface iseffectively prevented from being cut.

Subsequently, aluminum tapes (not shown) are applied to predeterminedpositions of the first and second abutment portions 48, 50 where theends 44 a, 46 a of the flange members 44, 46 are butted against the ends42 a, 42 b of the body member 42. Accordingly, the flange members 44, 46are temporarily joined on both sides of the body member 42 (Step S2).The body member 42 and the flange members 44, 46, which have beentemporarily joined to one another, are arranged in an unillustratedheating furnace, and heated to a predetermined temperature (Step S3).The body member 42 and the flange members 44, 46 heated to thepredetermined temperature are placed on the pedestal member 64 (StepS4).

Specifically, as shown in FIGS. 3 to 5, the first backing jig 88 isarranged around the ring section 86 on the support base 66 of thepedestal member 64. The bolts 96 are screwed from the back surface ofthe support base 66 into the screw holes 94 of the first backing jig 88.Accordingly, the first backing jig 88 is fixed on the support base 66(see FIGS. 4 and 5).

On the other hand, the screw sections 102 of the rods 90 are screwedinto the screw holes 100 formed in the ring section 86. The bolts 108are screwed into the screw holes 104 of the rods 90 through the holes106 of the second backing jig 92 while the second backing jig 92 isarranged on the rods 90. Thus, the second backing jig 92 is fixed ontothe respective rods 90. In this state, the body member 42 and the flangemembers 44, 46 heated to the predetermined temperature are externallyfitted on or attached to the first and second backing jigs 88, 92.

In this situation, according to the embodiment of the present invention,the body member 42 and the flange members 44, 46 are heated to thepredetermined temperature, and their inner circumferential diameters areexpanded by thermal expansion. Therefore, the first and second abutmentportions 48, 50 of the body member 42 and the flange members 44, 46 areexternally fitted on the first and second backing jigs 88, 92 with ease.When the body member 42 and the flange members 44, 46 are cooled, theinner circumferential diameters are shrunk or contracted. The innercircumferential surfaces 48 b, 50 b of the first and second abutmentportions 48, 50 are correctly fitted on and in close contact with theouter circumferential surfaces 88 a, 92 a of the first and secondbacking jigs 88, 92 (see FIG. 5). That is because the outer diameters ofthe outer circumferential surfaces 88 a, 92 a of the first and secondbacking jigs 88, 92 are larger than the inner diameters of the innercircumferential surfaces 48 b, 50 b of the first and second abutmentportions 48, 50 before being heated.

In this arrangement, the outer circumferential surfaces 88 a, 92 a ofthe first and second backing jigs 88, 92 are completely circular. Theends 42 a, 44 a and the ends 42 b, 46 a are maintained to have theidentical circumferential lengths, respectively, and are maintained tobe completely circular.

Subsequently, the pressing plate 74 is arranged on the second backingjig 92. The pressing block 77 is externally installed to the screwsection 72, and then the nut member 78 is screwed into the screw section72. Accordingly, the pressing plate 74 is pressed by the pressing block77, and the tightening load is applied in the directions of the arrow Ato the body member 42 and the flange members 44, 46. As a result, thefirst and second abutment portions 48, 50 are pressed and retained sothat no gap is formed (Step S5). When the process proceeds to Step S6,then the aluminum tapes (not shown) applied to the first and secondabutment portions 48, 50 are removed, and the surfaces are washed.

In Step S7, the first clamp jig 114 is attached. The first clamp jig 114is belt-shaped, and it circumscribes the outer circumference of the bodymember 42 along the side of the end 42 a. Accordingly, when the bolt 122is inserted into the hole 120, and the bolt 122 is screwed into thescrew hole 118, then the inner circumferential diameter of the firstclamp jig 114 is reduced. Therefore, the first clamp jig 114 tightensthe outer circumferential surface of the body member 42.

Subsequently, the pedestal member 64 is fixed by screws to the rotarytable 62 (Step S8). In this state, as shown in FIG. 7, the rotary tool142 of the welding machine 140 is moved toward the first abutmentportion 48 while rotating at a high speed. Accordingly, the probe 144rotated at the high speed is inserted into the first abutment portion48. The first abutment portion 48 is welded with frictional heat. Duringthis process, the rotary table 62 is rotated. The probe 144 rotated atthe high speed is moved relatively along the first abutment portion 48to join the entire circumference of the first abutment portion 48 (StepS9).

When the joining operation is completed for the first abutment portion48, the rotary table 62 is stopped, and the welding machine 140 isseparated from the first abutment portion 48. Further, the first clampjig 114 is detached, while the second clamp jig 116 is attached (StepS10). The first clamp jig 114 is removed from the body member 42 byincreasing the inner circumferential diameter by separating the bolt 122from the screw hole 118.

On the other hand, the plurality of rods 126 are attached to the outercircumferential edge portions of the support base 66 by the bolts 124.The attachment plate 130 is attached to the ends of the rods 126 by thebolts 128. The second clamp jig 116 is installed to the attachment plate130 by the bolts 132. The second clamp jig 116 tightens the outercircumferential surface on the side of the end 42 b of the body member42.

In this state, as shown in FIG. 8, for example, the welding machine 140faces the second abutment portion 50. While the probe 144 is rotatedtogether with the rotary tool 142, the probe 144 is inserted into theouter circumferential surface 50 a of the second abutment portion 50.The second abutment portion 50 is rotated with respect to the weldingmachine 140 together with the rotation of the rotary table 62. Theentire circumference of the second abutment portion 50 is subjected tothe friction stir welding (Step S11).

The first and second abutment portions 48, 50 are joined one another asdescribed above, and the duct structure 40 is obtained as a joinedproduct. After that, the duct structure 40 is detached from the pedestalmember 64 together with the first and second backing jigs 88, 92 (StepS12). Specifically, the bolts 112 are disengaged from the screw holes110 of the second backing jig 92, and the second backing jig 92 isreleased from the pressing plate 74. Further, when the bolts 108 aredisengaged from the screw holes 104 of the rods 90, the second backingjig 92 can be separated from the rods 90. On the other hand, when thebolts 96 are disengaged from the screw holes 94 of the first backing jig88, the first backing jig 88 can be separated from the support base 66.

Accordingly, the hanging bolt 79 and the nut member 78 are separatedfrom the screw section 72, and the pressing block 77 is detached.Further, the pressing plate 74 is detached from the support pillar 68.The duct structure 40 is removed from the support base 66 while thefirst and second backing jigs 88, 92 are fixed on the innercircumferential surface. The duct structure 40 is arranged in theunillustrated heating furnace to perform a heating treatment (Step S13).

In this embodiment, the duct structure 40 is made of the aluminum plate,while each of the first and second backing jigs 88, 92 is made of theiron-based material. Thus, thermal expansion is different from eachother. Therefore, the duct structure 40 is firstly thermally expanded inthe heating furnace. Accordingly, the first and second backing jigs 88,92 can be removed easily from the inner circumferential surface of theduct structure 40 (Step S14).

The hanging bolts 98, 113 are provided for the first and second backingjigs 88, 92. The hanging bolts 98, 113 are hung with an unillustratedcrane or the like. Accordingly, the first and second backing jigs 88, 92can be attached/detached with ease with respect to the pedestal member64.

As described above, in the embodiment of the present invention, theouter circumferential surfaces 48 a, 50 a of the first and secondabutment portions 48, 50 are welded by the friction stir welding whilethe inner circumferential surfaces 48 b, 50 b of the first and secondabutment portions 48, 50 are in close contact with the outercircumferential surfaces 88 a, 92 a of the first and second backing jigs88, 92 (see FIG. 7). Accordingly, when the probe 144 rotated at highspeed is moved relatively along each of the outer circumferentialsurfaces 48 a, 50 a while the probe 144 is inserted into each of theouter circumferential surfaces 48 a, 50 a of the first and secondabutment portions 48, 50, the first and second abutment portions 48, 50are not deformed.

In particular, the outer circumferential surfaces 88 a, 92 a of thefirst and second backing jigs 88, 92 are completely circular. The innercircumferential surfaces 48 b, 50 b of the first and second abutmentportions 48, 50 become completely circular since the innercircumferential surfaces 48 b, 50 b are in close contact with the outercircumferential surfaces 88 a, 92 a. Further, the respective ends 42 a,44 a have the identical inner circumferential length, and the ends 42 b,46 a have the identical circumferential length.

Therefore, even when the first and second abutment portions 48, 50 arethin and relatively large in diameter, phase difference is notgenerated, which would be otherwise caused, for example, by deformationor wrinkles. It is possible to maintain the satisfactory circularity forthe first and second abutment portions 48, 50, and the dimensionalaccuracy is improved. Accordingly, the friction stir welding operationis efficiently performed for the first and second abutment portions 48,50 by simple and economical steps.

The first and second abutment portions 48, 50 are fitted on and in closecontact with the outer circumferential surfaces 88 a, 92 a of the firstand second backing jigs 88, 92. Accordingly, it is possible to positionthe first and second abutment portions 48, 50 without deviation, and thefriction stir welding process is efficiently carried out. In thisprocedure, the first and second abutment portions 48, 50 are fitted onthe first and second backing jigs 88, 92 while the inner circumferentialdiameters are increased by thermal expansion by being heated to thepredetermined temperature. Therefore, it is possible to make the firstand second abutment portions 48, 50 closely contact the outercircumferential surfaces 88 a, 92 a of the first and second backing jigs88, 92 easily and correctly.

On the other hand, when the duct structure 40 is heated to increase theinner circumferential diameter after completing the joining process forthe first and second abutment portions 48, 50, the operation forseparating the first and second backing jigs 88, 92 is performed withease. Accordingly, it is easy to efficiently perform the entire processof the friction stir welding.

The pressing force is applied to the body member 42 and the flangemembers 44, 46 in the direction (directions of the arrow A)substantially perpendicular to the direction of insertion (direction ofthe arrow B) of the probe 144 by the pressing mechanism 70. Accordingly,the joining of the first and second abutment portions 48, 50 can becorrectly achieved by the pressure without a gap in the first and secondabutment portions 48, 50. The highly accurate joining process is carriedout. Further, the structure of the pressing mechanism 70 is simplified.

When the joining process is performed for the first abutment portion 48,the first clamp jig 114 is installed to the outer circumferentialsurface 48 a of the first abutment portion 48. Therefore, even when thefirst abutment portion 48 is expanded during the friction stir welding,the position of the first abutment portion 48 is not deviated, andhighly accurate joining operation is carried out. On the other hand,when the second abutment portion 50 is joined, the outer circumferentialsurface 50 a of the second abutment portion 50 is retained by the secondclamp jig 116, in which the same effect is obtained.

In the embodiment of the present invention, the flange members 44, 46and the body member 42 temporarily joined with the unillustratedaluminum tapes are installed to the first and second backing jigs 88, 92by increasing the inner circumferential diameters by effecting theheating to the predetermined temperature in the heating furnace.However, the invention is not limited thereto. Alternatively, forexample, the first and second backing jigs 88, 92 may be cooled todecrease the outer diameters of the first and second backing jigs 88,92.

In the present invention, the ends of the first and second cylindricalmembers are butted together at the abutment portion, and the abutmentportion has the second surface which is in close contact with the outercircumferential surface of the backing jig. The rotating probe ispressed against the first surface of the abutment portion to apply thejoining treatment thereby. Accordingly, even when the abutment portionis especially thin and large in diameter, it is possible to reliablyretain the abutment portion to have the desired shape. Therefore, thefriction stir welding operation is performed efficiently and highlyaccurately for the abutment portion by simple and economical arrangementand steps.

While the invention has been particularly shown and described withreference to preferred embodiments, it will be understood thatvariations and modifications can be effected thereto by those skilled inthe art without departing from the spirit and scope of the invention asdefined by the appended claims.

1. A friction stir welding apparatus for joining an abutment portion bymoving a rotating probe relatively along said abutment portion whilepressing said probe against one surface of said abutment portion atwhich an end of a first cylindrical member and an end of a secondcylindrical member are butted together, said friction stir weldingapparatus comprising: a pedestal member on which said first and secondcylindrical members are fixed; a backing jig on which said first andsecond cylindrical members are externally fitted so that the entirecircumference of another surface of said abutment portion has apredetermined circumferential length, said backing jig having adifferent thermal expansion from those of said first and secondcylindrical members, and being separated from said abutment portionafter friction stir welding; and a pressing mechanism which applies apressing force to said abutment portion, wherein said pressing mechanismincludes a pressing plate that applies said pressing force to said firstand second cylindrical members toward said pedestal member in adirection along a longitudinal axis of said first and second cylindermembers that is substantially perpendicular to a direction of insertionof said probe so as to abut said first and second cylindrical membersagainst each other, wherein an outer diameter of an outercircumferential surface of said backing jig is larger than a diameter ofsaid another surface of said abutment portion, and wherein said firstand second cylindrical members are externally fitted on said backing jigby relatively thermally expanding said first and second cylindricalmembers as compared with said backing jig.
 2. The friction stir weldingapparatus according to claim 1, wherein an outer circumferential surfaceof said backing jig has a completely circular shape, and circumferencesof said ends of said first and second cylindrical members, which are inclose contact with said outer circumferential surface, have an identicallength.
 3. The friction stir welding apparatus according to claim 1,further comprising a clamp jig which is arranged on said one surface ofsaid abutment portion and which prevents said abutment portion fromdeformation during said friction stir welding.
 4. A friction stirwelding apparatus for joining an abutment portion by moving a rotatingprobe relatively along said abutment portion while pressing said probeagainst one surface of said abutment portion at which an end of a firstcylindrical member and an end of a second cylindrical member are buttedtogether, said friction stir welding apparatus comprising: a pedestalmember on which said first and second cylindrical members are fixed,wherein said pedestal member rotates about a rotational axis; a backingjig on which said first and second cylindrical members are externallyfitted so that the entire circumference of another surface of saidabutment portion has a predetermined circumferential length, saidbacking jig having a different thermal expansion from those of saidfirst and second cylindrical members, and being separated from saidabutment portion after friction stir welding; and a pressing mechanismwhich applies a pressing force to said abutment portion, wherein thepressing mechanism includes a pressing plate that applies said pressingforce toward said pedestal member in a direction parallel to therotational axis of the pedestal member so as to abut said first andsecond cylindrical members against each other, wherein an outer diameterof an outer circumferential surface of said backing jig is larger than adiameter of said another surface of said abutment portion, and whereinsaid first and second cylindrical members are externally fitted on saidbacking jig by relatively thermally expanding said first and secondcylindrical members as compared with said backing jig.